Method and apparatus for lay flat control in an extruded film production line

ABSTRACT

An apparatus for producing an extruded film tube and supplying said tube to a collapsing and roller assembly includes a die for extruding a molten material in the form of a tube which is in a molten state below a frost line and in a solid state above the frost line. A blower system supplies and exhausts cooling air to and from an interior portion of the tube, and is regulated by a valve. At least two sensors are provided, one below the frost line for sensing the position of said tube, and one located proximate the tube in a position above said frost line. The upper sensor is used for sensing the position of the tube prior to collapsing and flattening it. A controller receives feedback signals from both sensors and controls operation of the valve.

PROVISIONAL PRIORITY CLAIM

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/176,457, filed Jan. 15, 2000, entitled “Methodand Apparatus for Lay Flat Control in an Extruded Film Production,”naming as sole inventor Daniel R. Joseph. This provisional applicationis incorporated herein as if fully set forth.

FIELD OF THE INVENTION

The present invention relates in general to an extruded film processingsystem, and in particular to control systems utilized in extruded filmprocessing systems.

DESCRIPTION OF THE PRIOR ART

Blown film extrusion lines are used to manufacture plastic bags andplastic sheets. A molten tube of plastic is extruded from an annulardie, and then stretched and expanded to a larger diameter and a reducedradial thickness by the action of overhead nip rollers and internal airpressure. Typically, ambient air is entrained by one or more blowers.The ambient air provides a cooling medium, which absorbs heat from themolten material. This speeds up the change in state from a moltenmaterial back to a solid material.

Additionally, the ambient air entrained by the blowers is used toprovide air pressure, which is utilized to control the size andthickness of the film tube. One type of blown film extrusion lineutilizes air flow on the exterior surface of the film tube in order toabsorb heat. A different, and more modern, type of blown film extrusionline utilizes both an external flow of cooling air and an internal flowof cooling air in order to cool and size the film tube. Whether theblown film tube is cooled from the interior surface, the exteriorsurface, or some combination of the two, one common problem in blownfilm extrusion lines is that of obtaining precise control over thediameter of the extruded film tube. Tight control over the diameterensures uniform product dimensions, which includes the size of theextruded product, as well as the thickness of the plastic material.

Acoustic sensors may be utilized to gauge the diameter of the product.When such acoustic sensors are utilized, a feedback loop is establishedto alter dynamically one or more controllable variable of the process,such as blower speed, and/or temperature control over the cooling airstream.

SUMMARY OF THE INVENTION

It is one objective of the present invention to provide a substantiallyimproved ability to keep blown film product width within establishedspecifications. This invention provides improved lay-flat control byadding a second feedback control loop, in addition to, and orsupplementation of, the primary control feedback loop which is utilizedto control the extrusion and cooling process.

This additional and/or supplemental control loop of the presentinvention measures actual bubble diameter, preferably (but notnecessarily) utilizing acoustic sensors, and feeds back this informationto one or more controllers. Preferably the controller is the one whichis utilized to perform the calculations and control operations of theprimary control loop for expanding and cooling the extruded film tube.The sensed diameter data is compared against an operator established setpoint. In the preferred embodiment, the resulting error is injected intothe Internal Bubble Cooling system (the “IBC”) to provide a correctioneffect. In the preferred embodiment, this is in fact directly added asan input to the primary control loop.

Preferably one or more non-contact acoustic sensors are located abovethe so-called “frost line”, thus providing a measure of the diameter ofthe product after cooling but preferably BEFORE flattening of theextruded film tube by an assembly of collapsing boards and nip rollers.In most conventional blown film lines, this assembly is located overheadof the die and related components. Thus the diameter sensors of thepresent invention are located above the sensors of the primary controlloop for controlling product diameter (through control of the expansionand cooling of the extruded film tube) but beneath the collapsing boardsand nip rollers. This preferred placement of the second set of bubblediameter measuring devices of the present invention above the IBCsensors provides a quicker response than established methods in theprior art. A variety of alternative sensors may be utilized in lieu ofan acoustic sensor. For example, mechanical feeler arms may be utilized,especially if the sensor is located sufficiently far from the frost lineto minimize the chance of creating deformations in the product throughcontact with the mechanical feeler arms. As a particular matter, anacoustic sensor works fine since it has no moving parts and creates nopressure on the tube or bubble. It may however be difficult (but notimpossible) to use optical sensor since the sensor response would bedependent on the color of the extruded tube. Accordingly, the preferredsensor is any non-optical sensor.

The prior art approach is characterized by the utilization of a lay-flatmeasuring bar after the primary nip rollers. In the prior art systems,the distance between the IBC sensors (of the primary control loop) andthe lay-flat bar can be nearly 40 feet and when oscillating nip devicesare used; of course, this path length of the prior art approach can varyas the nip oscillates.

One additional problem of the prior art is resolved by the presentinvention. IBC performance depends on stable airflow sources to maintaina stable bubble. Therefore, disturbances can result in changes in thefinal product width. In particular, rotating or oscillating dies usemoving air chambers that can induce a disturbance in the airflow as aresult of uneven airflow in the chamber. In the present invention, thevariation in product diameter resulting from the airflow changes thatoccur because of imbalances in the rotating chamber can be significantlyreduced.

In accordance with the preferred embodiment of the present invention,one or more sensors are positioned in a different horizontal plane fromthe IBC control sensors. Preferably, these sensors are also placed in adifferent circumferential position than the primary control loopsensors. In this patent, these sensors are called “lay-flat” sensors todistinguish them from the IBC sensors. In the preferred embodiment, theplacing the lay-flat sensors in a horizontal plane vertically above theIBC sensors provides optimum results. The purpose of these sensors is toprovide a measurement of the actual bubble diameter from which the finallay-flat dimension can be calculated from a simple formula (lay-flatequals pi multiplied by the sensed diameter divided by two).

The preferred system of the present invention monitors the sensor(s) forproper operation and selects which particular sensors are allowed tocontribute to the bubble diameter measurement. It also provides anindicator when all sensors are not allowed to contribute. The systemfilters the received signal from one or more sensors and calculates theexpected lay-flat.

This system can also accept a calibration input from the operator. Thiscalibration input allows the operator to indicate the current actuallay-flat as measured at the point of accumulation (such as a spoolingsystem) for the material. The system takes this reading and backcalculates an adjustment factor that accounts for the “draw down” of thematerial.

Draw down is the amount the material shrinks in width as a result of thetension placed on the material during accumulation. The amount of drawdown is dependent upon both the material utilized in the extrusion lineand the amount of tension utilized in the accumulation operations. Thusthe amount of “draw down” is a function of both material and tension.The mixture and composition of the material input into the blown filmline is relatively fixed for each product run; however, the material canvary greatly in composition (and associated physical properties) betweenproduct runs. The amount of tension applied to the accumulation orspooling system also varies between production lines and productionruns; however, the amount of tension applied is susceptible to a greateramount or range of operator (and computer-system) control.

Accordingly the lay-flat feature of the present invention is useful overa wide variety of materials, which are used in blown film line, and itis also useful over a wide range of production equipment.

In accordance with the preferred embodiment of the present invention,the system converts the actual lay-flat signal into a signal thatmatches the signal type used by the IBC sensor; in other words, thelay-flat signal can be translated to the units and scale utilized by theprimary control loop. The system directly accepts as an input theconverted lay-flat signal and compares it to the operator-establishedset point.

The system also monitors the signal rate of change and position againstoperator set windows of operation. This system essentially decides ifthe lay-flat signal is stable and within acceptable range for propercorrective action. If the signal is acceptable, the system applies anadjustable gain, inverts the signal and injects the signal into the IBCcontrol system.

The above as well as additional objectives, features, and advantageswill become apparent in the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself however, as well as apreferred mode of use, further objectives and advantages thereof, willbest be understood by reference to the following detailed description ofthe preferred embodiment when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a view of a blown film extrusion line equipped with theimproved control system of the present invention;

FIG. 2 is a view of the die, sizing cage, control subassembly androtating frame of the blown film tower of FIG. 1;

FIG. 3 is a view of the acoustic transducer of the improved controlsystem of the present invention coupled to the sizing cage of the blownfilm extrusion line tower adjacent the extruded film tube of FIGS. 1 and2;

FIG. 4 is a view of the acoustic transducer of FIG. 3 coupled to thesizing cage of the blown film tower, in two positions, one positionbeing shown in phantom;

FIG. 5A is a schematic and block diagram view of the preferred controlsystem of the present invention;

FIGS. 5B and 5C depict a bladder valve which may be utilized in lieu ofa rotary valve.

FIG. 6 is a schematic and block diagram view of the preferred controlsystem of FIG. 5, with special emphasis on the supervisory control unit;

FIG. 7A is a schematic and block diagram view of the signals generatedby the ultrasonic sensor which pertain to the position of the blown filmlayer;

FIG. 7B is a view of the ultrasonic sensor of FIG. 3 coupled to thesizing cage of the blown film tower, with permissible extruded film tubeoperating ranges indicated thereon;

FIG. 8A is a flow chart of the preferred filtering process applied tothe current position signal generated by the acoustic transducer;

FIG. 8B is a graphic depiction of the operation of the filtering system;

FIG. 9 is a schematic representation of the automatic sizing andrecovery logic (ASRL) of FIG. 6;

FIG. 10 is a schematic representation of the health/state logic (HSL) ofFIG. 6;

FIG. 11 is a schematic representation of the loop mode control logic(LMCL) of FIG. 6;

FIG. 12 is a schematic representation of the volume setpoint controllogic (VSCL) of FIG. 6;

FIG. 13 is a flow chart representation of the output clamp of FIG. 6;

FIG. 14 is a flow chart representation of the integration of the layflat control loop into an internal bubble control (IBC) system;

FIG. 15 is a block diagram representation of the combination of controlloops.

DETAILED DESCRIPTION OF THE INVENTION

Although the invention has been described with reference to a particularembodiment, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiments as well asalternative embodiments of the invention will become apparent to personsskilled in the art upon reference to the description of the invention.It is therefore contemplated that the appended clams will cover any suchmodifications or embodiments that fall within the scope of theinvention.

FIG. 1 is a view of blown film extrusion line 11, which includes anumber of subassemblies which cooperate to produce plastic bags and thelike from plastic resin. The main components include blown film tower13, which provides a rigid structure for mounting and aligning thevarious subassemblies, extruder subassembly 15, die subassembly 17,blower subassembly 19, stack 21, sizing cage 23, collapsible frame 25,nips 27, control subassembly 28 and rollers 29.

Plastic granules are fed into hopper 31 of extruder subassembly 15. Theplastic granules are melted and fed by extruder 33 and pushed into diesubassembly 17, and specifically to annular die 37. The molten plasticgranules emerge from annular die 37 as a molten plastic tube 39, whichexpands from the die diameter to a desired final diameter, which mayvary typically between two to three times the die diameter.

Blower subassembly 19 includes a variety of components which cooperatetogether to provide a flow of cooling air to the interior of moltenplastic tube 39, and also along the outer periphery of molten plastictube 39. Blower subassembly includes blower 41 which pulls air into thesystem at intake 43, and exhausts air from the system at exhaust 45. Theflow of air into molten plastic tube 39 is controlled at valve 47. Airis also directed along the exterior of molten plastic tube from externalair ring 49, which is concentric to annular die 37. Air is supplied tothe interior of molten plastic tube 39 through internal air diffuser 51.Air is pulled from the interior of molten plastic tube 39 by exhauststack 53.

The streams of external and internal cooling airs serve to harden moltenplastic tube 39 a short distance from annular die 37. The line ofdemarcation between the molten plastic tube 39 and the hardened plastictube 55 is identified in the trade as the “frost line.” Normally, thefrost line is substantially at or about the location at which the moltenplastic tube 39 is expanded to the desired final diameter.

Adjustable sizing cage 23 is provided directly above annular die 38 andserves to protect and guide the plastic tube 55 as it is drawn upwardthrough collapsible frame 25 by nips 27. Afterwards, plastic tube 55 isdirected through a series of rollers 57, 59, 61, and 63 which serve toguide the tube to packaging or other processing equipment.

In some systems, rotating frame 65 is provided for rotating relative toblown film tower 13. It is particularly useful in rotating mechanicalfeeler arms of the prior art systems around plastic tube 55 todistribute the deformations. Umbilical cord 67 is provided to allowelectrical conductors to be routed to rotating frame 65. Rotating frame65 rotates at bearings 71, 73 relative to stationary frame 69.

Control subassembly 28 is provided to monitor and control the extrusionprocess, and in particular the circumference of plastic tube 55. Controlsubassembly 28 includes supervisory control unit, and operator controlpanel 77.

FIG. 2 is a more detailed view of annular die 37, sizing cage 23,control subassembly 28, and rotating frame 65. As shown in FIG. 2,supervisory control unit 75 is electrically coupled to operator controlpanel 77, valve 47, and acoustic transducer 79. These componentscooperate to define the primary control loop for controlling the volumeof air contained within extruded film tube 81, and hence the thicknessand diameter of the extruded film tube 81. Valve 47 controls the amountof air directed by blower 41 into extruded film tube 81 through internalair diffuser 51.

If more air is directed into extruded film tube 81 by internal airdiffuser 51 than is exhausted from extruded film tube 81 by exhauststack 43, the circumference of extruded film tube 81 will be increased.Conversely, if more air is exhausted from the interior of extruded filmtube 81 by exhaust stack 53 than is inputted into extruded film tube 81by internal air diffuser 51, the circumference of extruded film tube 81will decrease.

In the preferred embodiment, flow control valve 47 is responsive tosupervisory control unit 75 for increasing or decreasing the flow of airinto extruded film tube 81. Operator control panel 77 serves to allowthe operator to select the diameter of extruded film tube 81. Acoustictransducer 79 serves to generate a signal corresponding to thecircumference of extruded film tube 81, and direct this signal tosupervisory control unit 75 for comparison to the circumference settingselected by the operator at operator control panel 77. This defines theprimary control loop.

If the actual circumference of extruded film tube 81 exceeds theselected circumference, supervisory control unit 75 operates flowcontrol valve 47 to restrict the passage of air from blower 41 intoextruded film tube 81. This results in a decrease in circumference ofextruded film tube 81. Conversely, if the circumference of extruded filmtube 81 is less than the selected circumference, supervisory controlunit 75 operates on flow control valve 47 to increase the flow of airinto extruded film tube 81 and increase its circumference. Of course,extruded film tube 81 will fluctuate in circumference, requiringconstant adjustment and readjustment of the inflow of air by operationof supervisory control unit 75 and flow control valve 47.

The view of FIG. 2 also depicts the secondary lay-flat control loop ofthe present invention which provides an additional and supplementalcontrol loop which provides information about the diameter of theextruded film tube as taken from a different portion of the extrudedfilm tube which is preferably above the frost line and which accordinglyprovides a different reading of the product diameter.

As is depicted in the view of FIG. 2, in the preferred embodiment, aplurality of non-contact sensors 501, 505, (preferably but notnecessarily acoustic sensors) are positioned adjacent the extruded filmtube vertically above the sensors of the primary control loop, in adifferent and higher horizontal plane. While optical and othernon-contact sensors could be utilized, acoustic sensors are preferred.

In this position, the second control loop provides information about thediameter of the product in the region between the primary IBC controlsensors and the collapsing frame 25 and roller nips 27. The preferredlocation of the lay-flat sensors is several feet above the IBC sensor,such as four to six feet above the IBC sensor. In comparison to thesystems of the prior art, this is advantageous since the prior artsystems measure the diameter thirty (30) to forty (40) feet away. Theeffective “time lag” associated with the present invention isnegligible, especially considering that production line speeds can rangefrom 20 feet per minute to 500 feet per minute. The “response time”associated with the present invention is essentially zero as compared tothe prior art systems, even at low production line speeds.

Preferably, the lay-flat sensors are secured through support assemblies503, 507 to a non-moving portion of the blown film tower 13, as opposedto the sizing cage 23 which moves relative to the tower 13. In thepreferred embodiment the lay-flat sensors are secured in a manner whichallows they to be moved inward and/or outward relative to the tower 13to place the sensors in sensing range of the expected bubble diameterfor the particular production run.

While the depiction of FIG. 2 shows the lay-flat sensors in the samecircumferential position as IBC sensor 89, but this is merely to easethe depiction of the entire system; preferably, the lay-flat sensors arelocated in a circumferential position other than in alignment with theIBC sensor, such as 90 degrees or 180 degrees shifted from thecircumferential position of the IBC sensor.

It is one objective of the present invention to provide a substantiallyimproved ability to keep blown film product width within establishedspecifications. This invention provides improved lay-flat control byadding a second feedback control loop, in addition to, and orsupplementation of, the primary control feedback loop which is utilizedto control the extrusion and cooling process.

This additional and/or supplemental control loop of the presentinvention measures actual bubble diameter, preferably (but notnecessarily) utilizing acoustic sensors, and feeds back this informationto one or more controllers. Preferably the controller is the one whichis utilized to perform the calculations and control operations of theprimary control loop for expanding and cooling the extruded film tube.The sensed diameter data is compared against an operator established setpoint. In the preferred embodiment, the resulting error is injected intothe Internal Bubble Cooling system (the “IBC”) to provide a correctioneffect. In the preferred embodiment, this is in fact directly added asan input to the primary control loop.

Preferably one or more non-contact acoustic sensors are located abovethe so-called “frost line”, thus providing a measure of the diameter ofthe product after cooling but preferably BEFORE flattening of theextruded film tube by an assembly of collapsing boards and nip rollers.In most conventional blown film lines, this assembly is located overheadof the die and related components. Thus the diameter sensors of thepresent invention are located above the sensors of the primary controlloop for controlling product diameter (through control of the expansionand cooling of the extruded film tube) but beneath the collapsing boardsand nip rollers. This preferred placement of the second set of bubblediameter measuring devices of the present invention above the IBCsensors provides a quicker response than established methods in theprior art. A variety of alternative sensors may be utilized in lieu ofan acoustic sensor. For example, mechanical feeler arms may be utilized,especially if the sensor is located sufficiently far from the frost lineto minimize the chance of creating deformations in the product throughcontact with the mechanical feeler arms. As a particular matter, anacoustic sensor works fine since it has no moving parts and creates nopressure on the tube or bubble. It may however be difficult (but notimpossible) to use optical sensor since the sensor response would bedependent on the color of the extruded tube. Accordingly, the preferredsensor is any non-optical sensor.

The prior art approach is characterized by the utilization of a lay-flatmeasuring bar after the primary nip rollers. In the prior art systems,the distance between the IBC sensors (of the primary control loop) andthe lay-flat bar can be nearly 40 feet and when oscillating nip devicesare used; of course, this path length of the prior art approach can varyas the nip oscillates.

One additional problem of the prior art is resolved by the presentinvention. IBC performance depends on stable airflow sources to maintaina stable bubble. Therefore, disturbances can result in changes in thefinal product width. In particular, rotating or oscillating dies usemoving air chambers that can induce a disturbance in the airflow as aresult of uneven airflow in the chamber. In the present invention, thevariation in product diameter resulting from the airflow changes thatoccur because of imbalances in the rotating chamber can be significantlyreduced.

In accordance with the preferred embodiment of the present invention,one or more sensors are positioned in a different horizontal plane fromthe IBC control sensors. Preferably, these sensors are also placed in adifferent circumferential position than the primary control loopsensors. In this patent, these sensors are called “lay-flat” sensors todistinguish them from the IBC sensors. In the preferred embodiment, theplacing the lay-flat sensors in a horizontal plane vertically above theIBC sensors provides optimum results. The purpose of these sensors is toprovide a measurement of the actual bubble diameter from which the finallay-flat dimension can be calculated from a simple formula (lay-flatequals pi multiplied by the sensed diameter divided by two).

The preferred system of the present invention monitors the sensor(s) forproper operation and selects which particular sensors are allowed tocontribute to the bubble diameter measurement. It also provides anindicator when all sensors are not allowed to contribute. The systemfilters the received signal from one or more sensors and calculates theexpected lay-flat.

This system can also accept a calibration input from the operator. Thiscalibration input allows the operator to indicate the current actuallay-flat as measured at the point of accumulation (such as a spoolingsystem) for the material. The system takes this reading and backcalculates an adjustment factor that accounts for the “draw down” of thematerial.

Draw down is the amount the material shrinks in width as a result of thetension placed on the material during accumulation. The amount of drawdown is dependent upon both the material utilized in the extrusion lineand the amount of tension utilized in the accumulation operations. Thusthe amount of “draw down” is a function of both material and tension.The mixture and composition of the material input into the blown filmline is relatively fixed for each product run; however, the material canvary greatly in composition (and associated physical properties) betweenproduct runs. The amount of tension applied to the accumulation orspooling system also varies between production lines and productionruns; however, the amount of tension applied is susceptible to a greateramount or range of operator (and computer-system) control.

Accordingly the lay-flat feature of the present invention is useful overa wide variety of materials, which are used in blown film line, and itis also useful over a wide range of production equipment.

In accordance with the preferred embodiment of the present invention,the system converts the actual lay-flat signal into a signal thatmatches the signal type used by the IBC sensor; in other words, thelay-flat signal can be translated to the units and scale utilized by theprimary control loop. The system directly accepts as an input theconverted lay-flat signal and compares it to the operator-establishedset point.

The system also monitors the signal rate of change and position againstoperator set windows of operation. This system essentially decides ifthe lay-flat signal is stable and within acceptable range for propercorrective action. If the signal is acceptable, the system applies anadjustable gain, inverts the signal and injects the signal into the IBCcontrol system.

FIG. 3 is a view of ultrasonic IBC sensor 89 of the improve controlsystem of the present invention coupled to sizing cage 23 adjacentextruded film tube 81. In the preferred embodiment, acoustic transducer79 comprises an ultrasonic measuring and control system manufactured byMassa Products Corporation of Hingham, Mass., Model Nos. E-369 andM5000, including a Massa Products ultrasonic sensor 89. It is anultrasonic ranging and detection device which utilizes high frequencysound waves which are deflected off objects and detected. In thepreferred embodiment, a pair of ultrasonic sensors 89 are used, one totransmit sonic pulses, and another to receive sonic pulses. For purposesof simplifying the description only one ultrasonic sensor 89 is shown,and in fact a single ultrasonic sensor can be used, first to transmit asonic pulse and then to receive the return in an alternating fashion.The elapsed time between an ultrasonic pulse being transmitted and asignificant echo being received corresponds to the distance betweenultrasonic sensor 89 and the object being sensed. Of course, thedistance between the ultrasonic sensor 89 and extruded film tube 81corresponds to the circumference of extruded film tube 81. In thepresent situation, ultrasonic sensor 89 emits an interrogatingultrasonic beam 87 substantially normal to extruded film tube 81 andwhich is deflected from the outer surface of extruded film tube 81 andsensed by ultrasonic sensor 89.

The M5000 sensor is actually a sensor with all the functions of theM4000, M450 and M410 combined. This means that the transmit, receive andtemperature compensation functions are all in a single 25×100 mm unit.It also includes a programmable on-board microprocessor that allows usto shift some of the signal filtering functions to the sensor. This isvery helpful as it allows us to free up the main controller forhigher-level tasks.

Similar acoustic or ultrasonic sensors can be utilized for the lay-flatsensors 501,505.

The Massa Products Corporation ultrasonic measurement and control systemincludes system electronics which utilize the duration of time betweentransmission and reception to produce a useable electrical output suchas a voltage or current. In the preferred embodiment, ultrasonic sensor89 is coupled to sizing cage 23 at adjustable coupling 83. In thepreferred embodiment, ultrasonic sensor 89 is positioned within seveninches of extruded film tube 81 to minimize the impact of ambient noiseon a control system. Ultrasonic sensor 89 is positioned so thatinterrogating ultrasonic beam 87 travels through a path which issubstantially normal to the outer surface of extruded film tube 81, tomaximize the return signal to ultrasonic sensor 89.

FIG. 4 is a view of ultrasonic sensor 89 of FIG. 3 coupled to sizingcage 23 of the blown film tower 13, in two positions, one position beingshown in phantom. In the first position, ultrasonic sensor 89 is shownadjacent extruded film tube 81 of a selected circumference. Whenextruded film tube 81 is downsized to a tube having a smallercircumference, ultrasonic sensor 89 will move inward and outwardrelative to the central axis of the adjustable sizing cage, along withthe adjustable sizing cage 23. The second position is shown in phantomwith ultrasonic sensor 89′ shown adjacent extruded film tube 81′ of asmaller circumference. For purposes of reference, internal air diffuser51 and exhaust stack 53 are shown in FIG. 4. The sizing cage is alsomovable upward and downward, so ultrasonic sensor 89 is also movableupward and downward relative to the frost line of the extruded film tube81.

FIG. 5A is a schematic and block diagram view of the preferred controlsystem of the present invention. The preferred acoustic transducer 79 ofthe present invention includes IBC ultrasonic sensor 89 and temperaturesensor 91 which cooperate to produce a current position signal which isindependent of the ambient temperature. IBC ultrasonic sensor 89 iselectrically coupled to ultrasonic electronics module 95, andtemperature sensor 91 is electrically coupled to temperature electronicsmodule 97. Together, ultrasonic electronics module 95 and temperatureelectronics module 97 comprise transducer electronics 93. Four signalsare produced by acoustic transducer 79, including one analog signal, andthree digital signals.

As shown in FIG. 5A, four conductors couple transducer electronics tosupervisory control unit 75. Specifically, conductor 99 routes a 0 to 10volts DC analog input to supervisory control unit 75. Conductors 101,103, and 105 provide digital signals to supervisory control unit 75which correspond to a target present signal, maximum override, andminimum override. These signals will be described below in greaterdetail.

Supervisory control unit 75 is electrically coupled to set point display109 through analog display output 107. An analog signal between 0 and 10volts DC is provided to set point display 109 which displays theselected distance between ultrasonic sensor 89 and extruded film tube81. A distance is selected by the operator through distance selector111. Target indicator 113, preferably a light, is provided to indicatethat the target (extruded film tube 81) is in range. Distance selector111 is electrically coupled to supervisory control unit 75 by distancesetting conductor 119. Target indicator 113 is electrically coupled tosupervisory control unit 75 through target present conductor 121.

Supervisory control unit 75 is also coupled via valve control conductor123 to proportional valve 125. In the preferred embodiment, proportionalvalve 125 corresponds to valve 47 of FIG. 1, and is a pressure controlcomponent manufactured by Proportionair of McCordsville, Ind., Model No.BBH. Proportional valve 125 translates an analog DC voltage provided bysupervisory control unit 75 into a corresponding pressure between 0.5and 1.2 bar. Proportional valve 125 acts on rotary valve 129 throughcylinder 127. Pressurized air is provided to proportional valve 125 frompressurized air supply 131 through 20 micron filter 133.

Also, as depicted in FIG. 5A, the lay-flat sensors dynamically provideunprocessed diameter measurements during blown film productionoperations to supervisory control unit 75, after signal filtering isperformed upon the raw measurements by signal filtering module 511, andscaling is performed by scaling module 513. The processed measurementdata is provided as an input to supervisory control unit 75 directly viaan input pin, or it is summed with the scaled diameter data on line 99.

FIG. 5B is a schematic and block diagram representation of an airflowcircuit for use in a blown film extrusion system which utilizes analternative to the rotary valve 129 of FIG. 5A. Input blower 613 isprovided to provide a supply of air which is routed into airflow circuit611. The air is received by conduit 615 and directed to airflow controldevice 617 of the present invention. Airflow control device 617 operatesas a substitute for a conventional rotary-type airflow valve 631, whichis depicted in simplified form also in FIG. 5B. The preferred airflowcontrol device 617 of the present invention is employed to increase anddecrease the flow of air to supply distributor box 619 which provides anair supply to annular die 621 from which blown film tube 623 extendsupward. Air is removed from the interior of blown film tube 623 byexhaust distributor box 625 which routes the air to conduit 627, andeventually to exhaust blower 629.

The preferred airflow control device 617 is depicted in fragmentarylongitudinal section view in FIG. 5C. As is shown, airflow controldevice 617 includes housing 635 which defines inlet 637 and outlet 639and airflow pathway 641 through housing 635. A plurality of selectivelyexpandable flow restriction members 671 are provided within housing 635in airflow pathway 641. In the view of FIG. 5C, selectively-expandableflow restriction members 673, 675, 677, 679, and 681 are depicted. Otherselectively-expandable flow restriction members are obscured in the viewof FIG. 5C. Manifold 685 is provided to route pressurized air to theinterior of selectively-expandable flow restriction members 671, andincludes conduit 683 which couples to a plurality of hoses, such 8 ashoses 687, 689, 691, 693, 695 which are depicted in FIG. 5C (other hosesare obscured in FIG. 5C).

Each of the plurality of selectively-expandable flow restriction membersincludes an inner air-tight bladder constructed of an expandablematerial such as an elastomeric material. The expandable bladder issurrounded by an expandable and contractible metal assembly. Preferably,each of the plurality of selective-expandable flow restriction membersis substantially oval in cross-section view (such as the view of FIG.5C), and traverse airflow pathway 641 across the entire width of airflowpathway 641. Air flows over and under each of the plurality ofselectively-expandable airflow restriction members, and each of themoperates as an choke to increase and decrease the flow of air throughhousing 635 as they are expanded and contracted. However, the flowrestriction is accomplished without creating turbulence in the airflow,since the selectively-actuable flow restriction members are foil shaped.

Returning now to FIG. 5A, airflow control device 617 is coupled toproportional valve 657 which receives either a current or voltagecontrol signal and selectively vents pressurized fluid to airflowcontrol device 617. In the preferred embodiment, proportional valve 657is manufactured by Proportion Air of McCordsville, Ind. Supply 651provides a source of pressurized air which is routed through pressureregulator 653 which maintains the pressurized air at a constant 30pounds per square inch of pressure. The regulated air is directedthrough filter 655 to remove dust and other particulate matter, and thenthrough proportional valve 657 to airflow control device 617.

In the preferred embodiment of the present invention, airflow controldevice 617 is manufactured by Tek-Air Systems, Inc. of Northvale, N.J.,and is identified as a “Connor Model No. PRD Pneumavalve”. This valve isthe subject matter of at least two U.S. patents, including U.S. Pat. No.3,011,518, which issued in December of 1961 to Day et al., and U.S. Pat.No. 3,593,645, which issued on Jul. 20, 1971, to Day et al., which wasassigned to Connor Engineering Corporation of Danbury, Conn., and whichis entitled “Terminal Outlet for Air Distribution” both of which areincorporated herein by reference as if fully set forth.

Use has revealed that this type of airflow control device provides forgreater control than can be provided by rotary type valve 631 (depictedin FIG. 5A for comparison purposes only), and is especially good atproviding control in mismatched load situations which would ordinarilybe difficult to control economically with a rotary type valve.

A number of airflow control devices like airflow control device 617 canbe easily coupled together in either series or parallel arrangement tocontrol the total volume of air provided to a blown film line or toallow economical load matching. In FIG. 5A, a series and a parallelcoupling of airflow control devices is depicted in phantom, with airflowcontrol devices 681, 683, and 685 coupled together with airflow controldevice 617. As shown in the detail airflow control device 617 is inparallel with airflow control device 683 but is in series communicationwith airflow control device 685. Airflow control device 685 is inparallel communication with airflow control device 681. Airflow controldevices 681 and 683 are in series communication.

FIG. 6 is a schematic and block diagram view of the preferred controlsystem of FIG. 5, with special emphasis on the supervisory control unit75 and the manner in which it processes the IBC sensor data. Extrudedfilm tube 81 is shown in cross-section with ultrasonic sensor 89adjacent its outer wall. Ultrasonic sensor 89 emits interrogating pulseswhich are bounced off of extruded film tube and sensed by ultrasonicsensor 89. The time delay between transmission and reception of theinterrogating pulse is processed by transducer electronics 93 to producefour outputs: CURRENT POSITION signal which is provided to supervisorycontrol unit 75 via analog output conductor 99, digital TARGET PRESENTsignal which is provided over digital output 105, a minimum overridesignal (MIO signal) indicative of a collapsing or undersized bubblewhich is provided over digital output conductor 103, and maximumoverride signal (MAO signal) indicative of an overblown extruded filmtube 81 which is provided over a digital output conductor 101.

As shown in FIG. 6, the position of extruded film tube 81 relative toultrasonic sensor 89 is analyzed and controlled with reference to anumber of distance thresholds and setpoints, which are shown in greaterdetail in FIG. 7A. All set points and thresholds represent distancesfrom reference R. The control system of the present invention attemptsto maintain extruded film tube 81 at a circumference which places thewall of extruded film tube 81 at a tangent to the line established byreference A. The distance between reference R and set point A may beselected by the user through distance selector 111. This allows the userto control the distance between ultrasonic sensor 89 and extruded filmtube 81.

The operating range of acoustic transducer 79 is configurable by theuser with settings made in transducer electronics 93. In the preferredembodiment, using the Massa Products transducer, the range of operationof acoustic transducer 79 is between 3 to 24 inches. Therefore, the usermay select a minimum circumference threshold C and a maximumcircumference threshold B, below and above which an error signal isgenerated. Minimum circumference threshold C may be set by the user at adistance d3 from reference R. Maximum circumference threshold B may beselected by the user to be a distance d2 from reference R. In thepreferred embodiment, setpoint A is set a distance of 7 inches fromreference R. Minimum circumference threshold C is set a distance of10.8125 inches from reference R. Maximum circumference threshold B isset a distance of 4.1 inches from reference R. Transducer electronics 93allows the user to set or adjust these distances at will provided theyare established within the range of operation of acoustic transducer 79,which is between 3 and 24 inches.

Besides providing an analog indication of the distance betweenultrasonic sensors 89 and extruded film tube 81, transducer electronics93 also produces three digital signals which provide informationpertaining to the position of extruded film tube 81. If extruded filmtube 81 is substantially normal and within the operating range ofultrasonic sensor 89, a digital “1” is provided at digital output 105.The signal is representative of a TARGET PRESENT signal. If extrudedfilm tube 81 is not within the operating range of ultrasonic sensor 89or if a return pulse is not received due to curvature of extruded filmtube 81, TARGET PRESENT signal of digital output 105 is low. Asdiscussed above, digital output 103 is a minimum override signal MIO. Ifextruded film tube 81 is smaller in circumference than the referenceestablished by threshold C, minimum override signal MIO of digitaloutput 103 is high. Conversely, if circumference of extruded film tube81 is greater than the reference established by threshold C, the minimumoverride signal MIO is low.

Digital output 101 is for a maximum override signal MAO. If extrudedfilm tube 81 is greater than the reference established by threshold B,the maximum override signal MAO is high. Conversely, if thecircumference of extruded film tube 81 is less than the referenceestablished by threshold B, the output of maximum override signal MAO islow.

The minimum override signal MIO will stay high as long as extruded filmtube 81 has a circumference less than that established by threshold C.Likewise, the maximum override signal MAO will remain high for as longas the circumference of extruded film tube 81 remains larger than thereference established by threshold B.

Threshold D and threshold E are also depicted in FIG. 7A. Threshold D isestablished at a distance d4 from reference R. Threshold E isestablished at a distance d5 from reference R. Thresholds D and E areestablished by supervisory control unit 75, not by acoustic transducer79. Threshold D represents a minimum circumference threshold forextruded film tube 81 which differs from that established by transducerelectronics 93. Likewise, threshold E corresponds to a maximumcircumference threshold which differs from that established by acoustictransducer 79. Thresholds D and E are established in the software ofsupervisory control unit 75, and provide a redundancy of control, andalso minimize the possibility of user error, since these threshold areestablished in software, and cannot be easily changed or accidentallychanged. The coordination of all of these thresholds will be discussedin greater detail below. In the preferred embodiment, threshold C isestablished at 10.8125 inches from reference R. Threshold E isestablished at 3.6 inches from reference R.

FIG. 7B is a side view of the ultrasonic sensor 89 coupled to sizingcage 23 of the blown film tower 13, with permissible extruded film tube81 operating ranges indicated thereon. Setpoint A is the desireddistance between ultrasonic sensor 89 and extruded film tube 81.Thresholds D and C are established at selected distances inward fromultrasonic sensor 89, and represent minimum circumference thresholds forextruded film tube 81. Thresholds B and E are established at selecteddistances from setpoint A, and establish separate maximum circumferencethresholds for extruded film tube 81. As shown in FIG. 7B, extruded filmtube 81 is not at setpoint A. Therefore, additional air must be suppliedto the interior of extruded film tube 81 to expand the extruded filmtube 81 to the desired circumference established by setpoint A.

If extruded film tube 81 were to collapse, two separate alarm conditionswould be registered. One alarm condition will be established whenextruded film tube 81 falls below threshold C. A second and separatealarm condition will be established when extruded film tube 81 fallsbelow threshold D. Extruded film tube 81 may also become overblown. Inan overblown condition, two separate alarm conditions are possible. Whenextruded film tube 81 expands beyond threshold B, an alarm condition isregistered. When extruded film tube 81 expands further to extend beyondthreshold E, a separate alarm condition is registered.

As discussed above, thresholds C and B are subject to user adjustmentthrough settings in transducer electronics 93. In contrast, thresholds Dand E are set in computer code of supervisory control unit 75, and arenot easily adjusted. This redundancy in control guards againstaccidental or intentional missetting of the threshold conditions attransducer electronics 93. The system also guards against thepossibility of equipment failure in transducer 79, or gradual drift inthe threshold settings due to deterioration, or overheating of theelectronic components contained in transducer electronics 93.

Returning now to FIG. 6, operator control panel 137 and supervisorycontrol unit 75 will be described in greater detail. Operator controlpanel 137 includes setpoint display 109, which serves to display thedistance d1 between reference R and setpoint A. Setpoint display 109includes a 7 segment display. Distance selector 111 is used to adjustsetpoint A. Holding the switch to the “+” position increases thecircumference of extruded film tube 81 by decreasing distance d1 betweensetpoint A and reference R. Holding the switch to the “−” positiondecreases the diameter of extruded film tube 81 by increasing thedistance between reference R and setpoint A.

Target indicator 113 is a target light which displays informationpertaining to whether extruded film tube 81 is within range ofultrasonic transducer 89, whether an echo is received at ultrasonictransducer 89, and whether any alarm condition has occurred. Blowerswitch 139 is also provided in operator control panel 137 to allow theoperator to selectively disconnect the blower from the control unit. Asshown in FIG. 6, all these components of operator control panel 137 areelectrically coupled to supervisory control unit 75.

Supervisory control unit 75 responds to the information provided byacoustic transducer 79, and operator control panel 137 to actuateproportional valve 125. Proportional valve 125 in turn acts uponpneumatic cylinder 127 to rotate rotary valve 129 to control the airflow to the interior of extruded film tube 81.

With the exception of analog to digital converter 141, digital to analogconverter 143, and digital to analog converter 145 (which are hardwareitems), supervisory control unit 75 is a graphic representation ofcomputer software resident in memory of supervisory control unit 75. Inthe preferred embodiment, supervisory control unit 75 comprises anindustrial controller, preferably a brand industrial controller ModelNo. T6000. Therefore, supervisory control unit 75 is essentially arelatively low-powered computer which is dedicated to a particular pieceof machinery for monitoring and controlling. In the preferredembodiment, supervisory control unit 75 serves to monitor many otheroperations of blown film extrusion line 11. The gauging and control ofthe circumference of extruded film tube 81 through computer software isone additional function which is “piggybacked” onto the industrialcontroller. Alternately, it is possible to provide an industrialcontroller or microcomputer which is dedicated to the monitoring andcontrol of the extruded film tube 81. Of course, dedicating amicroprocessor to this task is a rather expensive alternative.

For purposes of clarity and simplification of description, the operationof the computer program in supervisory control unit 75 have beensegregated into operational blocks, and presented as an amalgamation ofdigital hardware blocks. In the preferred embodiment, these softwaresubcomponents include: software filter 149, health state logic 151,automatic sizing and recovery logic 153, loop mode control logic 155,volume setpoint control logic 157, and output clamp 159. These softwaremodules interface with one another, and to PI loop program 147 ofsupervisory control unit 75. PI loop program is a software routineprovided in the Control Microsystems T6000 system. The proportionalcontroller regulates a process by manipulating a control element throughthe feedback of a controlled output. The equation for the output of a PIcontroller is:

m=K*e+K/T∫e dt+ms

In this equation:

m=controller output

K=controller gain

e=error

T=reset time

dt=differential time

ms=constant

e dt=integration of all previous errors

When an error exists, it is summed (integrated) with all the previouserrors, thereby increasing or decreasing the output of the PI controller(depending upon whether the error is positive or negative). Thus as theerror term accumulates in the integral term, the output changes so as toeliminate the error.

CURRENT POSITION signal is provided by acoustic transducer 79 via analogoutput 99 to analog to digital converter 141, where the analog CURRENTPOSITION signal is digitized. The digitized CURRENT POSITION signal isrouted through software filter 149, and then to PI loop program 147. Ifthe circumference of extruded film tube 81 needs to be adjusted, PI loopprogram 147 acts through output clamp 159 upon proportional valve 125 toadjust the quantity of air provided to the interior of extruded filmtube 81.

FIG. 8A is a flowchart of the preferred filtering process applied toCURRENT POSITION signal generated by the acoustic transducer. Thedigitized CURRENT POSITION signal is provided from analog to digitalconverter 141 to software filter 149. The program reads the CURRENTPOSITION signal in step 161. Then, the software filter 149 sets SAMPLE(N) to the position signal.

In step 165, the absolute value of the difference between CURRENTPOSITION (SAMPLE (N)) and the previous sample (SAMPLE (N−1)) is comparedto a first threshold. If the absolute value of the difference betweenthe current sample and the previous sample is less than first thresholdT1, the value of SAMPLE (N) is set to CFS, the current filtered sample,in step 167. If the absolute value of the difference between the currentsample and the previous sample exceeds first threshold T1, in step 169,the CURRENT POSITION signal is disregarded, and the previous positionsignal SAMPLE (N−1) is substituted in its place.

Then, in step 171, the suggested change SC is calculated, by determiningthe difference between the current filtered sample CFS and the bestposition estimate BPE. In step 173, the suggested change SC which wascalculated in step 171 is compared to positive T2, which is the maximumlimit on the rate of change. If the suggested change is within themaximum limit allowed, in step 177, allowed change AC is set to thesuggested change SC value. If, however, in step 173, the suggestedchange exceeds the maximum limit allowed on the rate of change, in step175, the allowed change is set to +LT2, a default value for allowedchange.

In step 179, the suggested change SC is compared to the negative limitfor allowable rates of change, negative T2. If the suggested change SCis greater than the maximum limit on negative change, in step 181,allowed change AC is set to negative −LT2, a default value for negativechange. However, if in step 179 it is determined that suggested changeSC is within the maximum limit allowed on negative change, in step 183,the allowed change AC is added to the current best position estimateBPE, in step 183. Finally, in step 185, the newly calculated bestposition estimate BPE is written to the PI loop program.

Software filter 149 is a two stage filter which first screens theCURRENT POSITION signal by comparing the amount of change, eitherpositive or negative, to threshold T1. If the CURRENT POSITION signal,as compared to the preceding position signal exceeds the threshold ofT1, the current position signal is discarded, and the previous positionsignal (SAMPLE (N−1)) is used instead. At the end of the first stage, instep 171, a suggested change SC value is derived by subtracting the bestposition estimate BPE from the current filtered sample CFS.

In the second stage of filtering, the suggested change SC value iscompared to positive and negative change thresholds (in steps 173 and179). If the positive or negative change thresholds are violated, theallowable change is set to a preselected value, either +LT2, or −LT2. Ofcourse, if the suggested change SC is within the limits set by positiveT2 and negative T2, then the allowable change AC is set to the suggestedchange SC.

The operation of software filter 149 may also be understood withreference to FIG. 8B. In the graph of FIG. 8B, the y-axis represents thesignal level, and the x-axis represents time. The signal as sensed byacoustic transducer 79 is designated as input, and shown in the solidline. The operation of the first stage of the software filter 149 isdepicted by the current filtered sample CFS, which is shown in the graphby cross-marks. As shown, the current filtered sample CFS operates toignore large positive or negative changes in the position signal, andwill only change when the position signal seems to have stabilized for ashort interval. Therefore, when changes occur in the current filteredsample CFS, they occur in a plateau-like manner.

In stage two of the software filter 149, the current filtered sample CFSis compared to the best position estimate BPE, to derive a suggestedchange SC value. The suggested SC is then compared to positive andnegative thresholds to calculate an allowable change AC which is thenadded to the best position estimate BPE. FIG. 8B shows that the bestposition estimate BPE signal only gradually changes in response to anupward drift in the POSITION SIGNAL. The software filtering system 149of the present invention renders the control apparatus relativelyunaffected by random noise, but capable of tracking the more “gradual”changes in bubble position.

Experimentation has revealed that the software filtering system of thepresent invention operates best when the position of extruded film tube81 is sampled between 20 to 30 times per second. At this sampling rate,one is less likely to incorrectly identify noise as a change incircumference of extruded film tube 81. The preferred sampling rateaccounts for the common noise signals encountered in blown filmextrusion liner.

Optional thresholds have also been derived through experimentation. Inthe first stage of filtering, threshold T1 is established as roughly onepercent of the operating range of acoustic transducer 79, which in thepreferred embodiment is twenty-one meters (24 inches less 3 inches). Inthe second stage of filter, thresholds +LT2 and −LT2 are established asroughly 0.30% of the operating range of acoustic transducer 79.

FIG. 9 is a schematic representation of the automatic sizing andrecovery logic ASRL of supervisory control unit 75. As stated above,this figure is a hardware representation of a software routine. ASRL 153is provided to accommodate the many momentary false indications ofmaximum and minimum circumference violations which may be registered dueto noise, such as the noise created due to air flow between acoustictransducer 79 and extruded film tube 81. The input from maximum alarmoverride MAO is “ored” with high alarm D, from the PI loop program, at“or” operator 191. High alarm D is the signal generated by the programin supervisory control unit 75 when the circumference of extruded filmtube 81 exceeds threshold D of FIG. 7A. If a maximum override MAO signalexists, or if a high alarm condition D exists, the output of “or”operator 191 goes high, and actuates delay timer 193.

Likewise, minimum override MIO signal is “ored” at “or” operator 195with low alarm E. If a minimum override signal is present, or if a lowalarm condition E exists, the output of “or” operator 195 goes high, andis directed to delay timer 197. Delay timers 193, 197 are provided toprevent an alarm condition unless the condition is held for 800milliseconds continuously. Every time the input of delay timers 193, 197goes low, the timer resets and starts from 0. This mechanism eliminatesmany false alarms.

If an alarm condition is held for 800 milliseconds continuously, anOVERBLOWN or UNDERBLOWN signal is generated, and directed to the healthstate logic 151. Detected overblown or underblown conditions are “ored”at “or” operator 199 to provide a REQUEST MANUAL MODE signal which isdirected to loop mode control logic 155.

FIG. 10 is a schematic representation of the health-state logic 151 ofFIG. 6. The purpose of this logic is to control the target indicator 113of operator control panel 137. When in non-error operation, the targetindicator 113 is on if the blower is on, and the TARGET PRESENT signalfrom digital output 105 is high. When an error is sensed in the maximumoverride MAO or minimum override MIO lines, the target indicator 113will flash on and off in one half second intervals.

In health-state logic HSL 151, the maximum override signal MAO isinverted at inverter 205. Likewise, the minimum override signal isinverted at inverter 207.

“And” operator 209 serves to “and” the inverted maximum override signalMAO, with the OVERBLOWN signal, and high alarm signal D. A high outputfrom “and” operator 209 indicates that something is wrong with thecalibration of acoustic transducer 79.

Likewise, “and” operator 213 serves to “and” the inverted minimumoverride signal MIO, with the OVERBLOWN signal, and low alarm signal E.If the output of “and” operator 213 is high, something is wrong with thecalibration of acoustic transducer 79. The outputs from “and” operators209, 213 are combined in “or” operator 215 to indicate an error witheither the maximum or minimum override detection systems. The output of“or” operator 215 is channeled through oscillator 219, and inverted atinverter 217. “And” operator 211 serves to “and” the TARGET PRESENTsignal, blower signal, and inverted error signal from “or” operator 215.The output of “and” operator of 211 is connected to target indicator113.

If acoustic transducer 79 is properly calibrated, the target is withinrange and normal to the sonic pulses, and the blower is on, targetindicator 113 will be on. If the target is within range and normal tothe sonic pulses, the blower is on, but acoustic transducer 79 is out ofcalibration, target indicator 113 will be on, but will be blinking. Theblinking signal indicates that acoustic transducer 79, and in particulartransducer electronics 93, must be recalibrated.

FIG. 11 is a schematic representation of loop mode control logic LMCL ofFIG. 6. The purpose of this software module is coordinate the transitionin modes of operation. Specifically, this software module coordinatesautomatic startup of the blown film extrusion process, as well aschanges in mode between an automated “cascade” mode and a manual mode,which is the required mode of the PI controller to enable under andoverblown conditions of the extruded film tube 81 circumference. Theplurality of input signals are provided to loop mode control logic 155,including: BLOWER ON, REQUEST MANUAL MODE, PI LOOP IN CASCADE MODE,UNDERBLOWN and OVERBLOWN. Loop mode control logic LMCL 155 provides twooutput signals: MANUAL MODE, and CASCADE MODE.

FIG. 11 includes a plurality of digital logic blocks which arerepresentative of programming operations. “Or” operator 225 “ores” theinverted BLOWER ON SIGNAL to the REQUEST MANUAL MODE SIGNAL. “And”operator 227 “ands” the inverted REQUEST MANUAL MODE SIGNAL with aninverted MANUAL MODE SIGNAL, and the BLOWER ON SIGNAL. “And” operator229 “ands” the REQUEST MANUAL MODE SIGNAL to the inverted CASCADE MODESIGNAL. This prevents MANUAL MODE and CASCADE MODE from both being on atthe same time. “And” operator 231 “ands” the MANUAL MODE SIGNAL, theinverted UNDERBLOWN SIGNAL, and the OVERBLOWN SIGNAL. “And” operator 233“ands” the MANUAL MODE SIGNAL with the UNDERBLOWN SIGNAL. This causesthe overblown condition to prevail in the event a malfunction causesboth underblown and overblown conditions to be on. Inverters 235, 237,239, 241, and 243 are provided to invert the inputted output signals ofloop mode control logic 155 were needed. Software one-shot 245 isprovided for providing a momentary response to a condition. Softwareone-shot 245 includes “and” operator 247, off-delay 249, and inverter251.

The software of loop mode control logic 155 operates to ensure that thesystem is never in MANUAL MODE, and CASCADE MODE at the same time. Whenmanual mode is requested by REQUEST MANUAL MODE, loop mode control logic155 causes MANUAL MODE to go high. When manual mode is not requested,loop mode control logic 155 operates to cause CASCADE MODE to go high.MANUAL MODE and CASCADE MODE will never be high at the same time. Loopmode control logic 155 also serves to ensure that the system provides a“bumpless transfer” when mode changes occur. The term “cascade mode” isunderstood in the automation industries as referring to an automaticmode which will read an adjustable setpoint.

Loop mode control logic 155 will also allow for automatic startup of theblown film extrusion process. At startup, UNDERBLOWN SIGNAL is high, PILOOP IN CASCADE MODE is low, BLOWER ON SIGNAL is high. These inputs (andinverted inputs) are combined at “and” operators 231, 233. At startup,“and” operator 233 actuates logic block 253 to move the maximum air flowvalue address to the PI loop step 261. At startup, the MANUAL MODESIGNAL is high. For the PI loop controller of the preferred embodiment,when MANUAL MODE is high, the value contained in PI loop output addressis automatically applied to proportional valve 125. This results inactuation of proportional valve 125 to allow maximum air flow to startthe extruded film tube 81.

When extruded film tube 81 extends in size beyond the minimum threshold(C and D of FIG. 7A), the UNDERBLOWN SIGNAL goes low, and the PI LOOP INCASCADE MODE signal goes high. This causes software one-shot 245 totrigger, causing logic blocks 265, 267 to push an initial bias valuecontained in a program address onto the PI loop. Simultaneously, logicblocks 269, 271 operate to place the selected setpoint value A ontovolume-setpoint control logic VSCL 157. Thereafter, volume-setpointcontrol logic VSCL 157 alone serves to communicate changes in setpointvalue A to PI loop program 147.

If an overblown or underblown condition is detected for a sufficientlylong period of time, the controller will request a manual mode bycausing REQUEST MANUAL MODE SIGNAL to go high. If REQUEST MANUAL MODEgoes high, loop mode control logic LMCL 155 supervises the transferthrough operation of the logic blocks.

Loop mode control logic LMCL 155 also serves to detected overblown andunderblown conditions. If an overblown or underblown condition isdetected by the control system, REQUEST MANUAL MODE goes high, and theappropriate OVERBLOWN or UNDERBLOWN signal goes high. The logicoperators of loop mode control logic LMCL 155 operate to override thenormal operation of the control system, and cause maximum or minimum airflow by putting the maximum air flow address 261 or minimum air flowaddress 263 to the PI output address. As stated above, when MANUAL MODEis high, these maximum or minimum air flow address values are outputteddirectly to proportional valve 125. Thus, when the extruded film tube 81is overblown, loop mode control logic LMCL 155 operates to immediatelycause proportional valve 125 to minimize air flow to extruded film tube81. Conversely, if an underblown condition is detected, loop modecontrol logic LMCL 155 causes proportional valve 125 to immediatelymaximize air flow to extruded film tube 81.

FIG. 12 depicts the operation of volume-setpoint control logic VSCL 157.

Volume setpoint control logic VSCL 157 operates to increase or decreasesetpoint A in response to changes made by the operator at distanceselector 111 of operator control panel 137, when the PI loop program 147is in cascade mode, i.e. when PI LOOP IN CASCADE MODE signal is high.The INCREASE SETPOINT, DECREASE SETPOINT, and PI LOOP IN CASCADE MODEsignals are logically combined at “and” operators 283, and 287. These“and” operators act on logic blocks 285, 289 to increase or decrease thesetpoint contained in remote setpoint address 291. When the setpoint iseither increased or decreased, logic block 293 operates to add theoffset to the remote setpoint for display, and forwards the informationto digital to analog converter 143, for display at setpoint display 109of operator control panel 137. The revised remote setpoint address isthen read by the PI loop program 147.

FIG. 13 is a flowchart drawing of output clamp 159. The purpose of thissoftware routine is to make sure that the PI loop program 147 does notover drive the rotary valve 129 past a usable limit. Rotary valve 129operates by moving a vane to selectively occlude stationary openings. Ifthe moving vane is over driven, the rotary valve will begin to open whenthe PI loop calls for complete closure. In step 301, the output of thePI loop program 147 is read. In step 303, the output of PI loop iscompared to a maximum output. If it exceeds the maximum output, the PIoutput is set to a predetermined maximum output in step 305. If theoutput of PI loop does not exceed the maximum output, in step 307, theclamped PI output is written to the proportional valve 125 throughdigital to analog converter 145.

The operation of the lay-flat control loop will now be described withreference to FIGS. 14 and 15. FIG. 14 is a flow chart representation ofthe overall process of implementing the preferred lay-flat control loopin accordance with the preferred embodiment of the present invention.The process is a supplemental process to the primary IBC control loop.The computer implemented steps are executed utilizing the processorwhich is utilized for the IBC control loop. As is shown in FIG. 14, theprocess begins at block 601. In step 603, the processor determineswhether or not there is an automated measurement system for measuringthe width of the final product as accumulated or spooled. If such anautomated system exists, then control passes to block 605 wherein thewidth measure is read from the the automated system. If no suchautomated system exists, then control passes to block 607, wherein theoperator is prompted to enter the product width.

Once the product width information is obtained, in accordance with block609, the measure is loaded in memory. Then in accordance with block 611,the lay-flat measurement system is activated to provide dynamic and realtime information about the product diameter. In block 613, thecontroller determines whether or not the lay-flat sensors are in range.If the sensors are not in range, control passes to block 615 wherein theoperator is prompted to reposition the acoustic sensors so that they arein range. After repositioning is confirmed in block 617, control passesto block 623; however, if repositioning is not confirmed, then theprocess ends in accordance with block 619 and a warning is given inaccordance with block 621. Such warning can be a simple beeping sound ora blinking light, whatever is deemed sufficient to provide the operatorwith a warning.

Next in accord with block 623, the particular sensors which will beutilized are selected. Then in accordance with block 625, the processormonitors the output signals of all of the available sensors in order todetermine which signals are the most stable and reliable. Signal rate ofchange is a good way to identify the best sensors, with high rates ofchange indicating a poor sensor. Next the most reliable signals arecalibrated to match the scale of the signal provided to the controlsystem by the IBC sensor. Then in accordance with block 629, the errorsignal developed by the lay-flat sensors are injected into the feedbackloop in order to supplement the feed back loop of the IBC control loop.

FIG. 15 depicts the process in a high level block diagram. IBC sensormonitors bubble position 707 and provides a feed back signal tocontroller 703. Controller 703 supplies a control signal to valve 705.This will have an impact on the bubble position 707. In accordance withthe present invention, lay-flat sensor 711 monitors the diameter orwidth of the hardened product prior to collapsing and provides a similarinput to controller 703. Together the feed back signals form IBC sensor701 and lay-flat sensor 711 allows better and more timely control overthe diameter of the finished product than can be accomplished with theprior art approaches.

What is claimed is:
 1. An apparatus for producing an extruded film tubeand supplying said tube to a collapsing and roller assembly, comprising:(a) a die for extruding a molten material in the form of a tube which isin a molten state below a frost line and which is in a solid state abovesaid frost line; (b) a blower system for supplying and exhaustingcooling air to and from an interior portion of said tube; (c) a flowcontrol valve for regulating the at least a portion of said blowersystem to control the extrusion and cooling process, and whichdetermines in part the circumference of said tube; (d) at least onesizing sensor located proximate said tube in a position below said frostline for sensing the position of said tube, comparing such position toan extrusion set point, and generating an extrusion feedback errorsignal which is corrective of any difference between said position andsaid set point; (e) at least one lay flat sensor located proximate saidtube in a position above said frost line for sensing the position ofsaid tube prior to the collapsing and flattening of said tube by saidcollapsing and roller assembly, comparing said position to a lay-flatset point, and generating a lay-flat feedback error signal which iscorrective of any difference between said position and said lay-flat setpoint; (f) a programmable controller for executing program instructionsincluding a negative feedback control system which receives saidextrusion feedback error signal and said lay-flat feedback error signalas negative feedback injection signals and which provides a controlsignal to said valve.
 2. An apparatus according to claim 1, wherein saidat least one sizing sensor and said at least one lay flat sensor aremaintained in different circumferential positions relative to said tube.3. An apparatus according to claim 1, wherein said lay-flat feedbackerror signal is provided in the same units as said extrusion feedbackerror signal.
 4. An apparatus according to claim 1, wherein saidextrusion feedback error signal defines a primary control feedbackcontrol loop, and wherein said lay-flat feedback error signal defines asupplemental feedback control loop.
 5. An apparatus according to claim1, wherein said lay-flat feedback error signal is injected directly intosaid extrusion feedback control loop.
 6. An apparatus according to claim1, wherein said lay-flat feedback error signal is analyzed prior toinjection to determine if it is within a range of acceptable positions.7. An apparatus according to claim 1, wherein said lay-flat feedbackerror signal is analyzed prior to injection to determine if it is withinan acceptable range of signal rates of change.
 8. An apparatus forproducing an extruded film tube and supplying said tube to a collapsingand roller assembly, comprising: (a) a die for extruding a moltenmaterial in the form of a tube which is in a molten state below a frostline and in a solid state above said frost line; (b) a blower system forsupplying and exhausting cooling air to and from an interior portion ofsaid tube; (c) a valve for regulating the at least a portion of saidblower system to control the extrusion and cooling process, and whichdetermines in part the circumference of said tube; (d) at least oneacoustic sensor located proximate said tube in a position below saidfrost line for sensing the position of said tube, comparing suchposition to an extrusion set point, and generating an extrusion feedbackerror signal which is corrective of any difference between said positionand said set point; (e) at least one non-contact sensor locatedproximate said tube in a position above said frost line for sensing theposition of said tube prior to the collapsing and flattening of saidtube by said collapsing and roller assembly, comparing said position toa lay-flat set point, and generating a lay-flat feedback error signalwhich is corrective of any difference between said position and saidlay-flat set point; (f) a programmable controller for executing programinstructions including a negative feedback control system which receivessaid extrusion feedback error signal and said lay-flat feedback errorsignal as negative feedback injection signals and which provides acontrol signal to said valve.
 9. An apparatus according to claim 8,wherein said at least one acoustic sensor and said at least onenon-contact sensor are maintained in different circumferential positionsrelative to said tube.
 10. An apparatus according to claim 8, whereinsaid lay-flat feedback error signal is provided in the same units assaid extrusion feedback error signal.
 11. An apparatus according toclaim 8, wherein said extrusion feedback error signal defines a primarycontrol feedback control loop, and wherein said lay-flat feedback errorsignal defines a supplemental feedback control loop.
 12. An apparatusaccording to claim 8, wherein said lay-flat feedback error signal isinjected directly into said extrusion feedback control loop.
 13. Anapparatus according to claim 8, wherein said lay-flat feedback errorsignal is analyzed prior to injection to determine if it is within arange of acceptable positions.
 14. An apparatus according to claim 8,wherein said lay-flat feedback error signal is analyzed prior toinjection to determine if it is within an acceptable range of signalrates of change.
 15. A method of producing an extruded film tube andsupplying said tube to a collapsing and roller assembly, comprising: (a)extruding a molten material from a die in the form of a tube which is ina molten state below a frost line and in a solid state above said frostline; (b) utilizing a blower system for supplying and exhausting coolingair to and from an interior portion of said tube; (c) utilizing a flowcontrol valve for regulating the at least a portion of said blowersystem to control the extrusion and cooling process, and whichdetermines in part the circumference of said tube; (d) locating at leastone acoustic sensor proximate said tube in a position below said frostline for sensing the position of said tube; (e) comparing such positionto an extrusion set point; (f) generating an extrusion feedback errorsignal which is corrective of any difference between said position andsaid set point; (g) locating at least one non-contact sensor locatedproximate said tube in a position above said frost line for sensing theposition of said tube prior to the collapsing and flattening of saidtube by said collapsing and roller assembly; (h) comparing said positionto a lay-flat set point; (i) generating a lay-flat feedback error signalwhich is corrective of any difference between said position and saidlay-flat set point; (j) providing programmable controller for executingprogram instructions; (k) including in said programmable instructions anegative feedback control system which receives said extrusion feedbackerror signal and said lay-flat feedback error signal as negativefeedback injection signals and which provides a control signal to saidvalve.
 16. A method according to claim 15, further comprising: locatingsaid at least one acoustic sensor and said at least one non-contactsensor in different circumferential positions relative to said tube. 17.A method according to claim 15, wherein said lay-flat feedback errorsignal is provided in the same units as said extrusion feedback errorsignal.
 18. A method according to claim 15, wherein said extrusionfeedback error signal defines a primary control feedback control loop,and wherein said lay-flat feedback error signal defines a supplementalfeedback control loop.
 19. A method according to claim 15, wherein saidlay-flat feedback error signal is injected directly into said extrusionfeedback control loop.
 20. A method according to claim 15, wherein saidlay-flat feedback error signal is analyzed prior to injection todetermine if it is within a range of acceptable positions.
 21. A methodaccording to claim 15, wherein said lay-flat feedback error signal isanalyzed prior to injection to determine if it is within an acceptablerange of signal rates of change.